The present application claims priority to Japanese Application(s) No(s). P2001-158526 filed May 28, 2001, which application(s) is/are incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
The present invention relates to a liquid crystal display device and to a projection liquid crystal display apparatus which displays an image by using the liquid crystal display device.
2. Description of the Related Art
Hitherto, projection liquid crystal display apparatuses (liquid crystal projectors) which project light modulated by liquid crystal display devices (hereinafter referred to as liquid crystal panels) on a screen and thereby display an image on the screen are known in the art. There are two types of image-projection methods used in projection liquid crystal display apparatuses: a front projection type (front type) in which an image is projected onto the screen from the front side of a screen, and a rear projection type (rear type) in which an image is projected onto a screen from the rear side of the screen. In addition, there are two types of projection liquid crystal display apparatuses for displaying color images: a single-panel type in which a single liquid crystal panel is used and a three-panel type in which three liquid crystal panels for three colors, that is, red (R), green (G), and blue (B), are used.
FIG. 12 is a schematic diagram showing an optical system (mainly an illuminating optical system) of a projection liquid crystal display apparatus of the known art. In this projection liquid crystal display apparatus, a light source 101, first and second multi-lens array integrators (hereinafter abbreviated as MLAs) 102 and 103 forming a pair, a PS composite element 104, a condenser lens 105, a field lens 106, a liquid crystal panel 107, and a projection lens 108 are arranged along an optical axis 100. The MLAs 102 and 103 include a plurality of small lenses (microlenses) 102M and 103M, respectively, which are arranged two-dimensionally. The PS composite element 104 includes a plurality of half-wave plates 104A at positions corresponding to the positions between adjacent microlenses on the second MLA 103.
In this projection liquid crystal display apparatus, illuminating light emitted from the light source 101 is divided into a plurality of light beams when it passes through the MLAs 102 and 103. The light beams emitted from the MLAs 102 and 103 are incident on the PS composite element 104. Light L10, which is incident on the PS composite element 104 includes a P-polarized light component and a S-polarized light component which intersect each other on a plane perpendicular to the optical axis 100. The PS composite element 104 serves to separate the incident light L10 into the two kinds of polarized light components (a P-polarized light component and an S-polarized light component) L11 and L12. After the polarized light components L11 and L12 are separated from each other, the light component L11 leaves the PS composite element 104 without changing its polarization direction (for example, the P-polarization). Conversely, the polarization direction of the light component L12 (for example, the S-polarization) is changed to the other direction (for example, the P-polarization) by the half-wave plate 104A upon exiting the PS composite element 104. Accordingly, light having a predetermined polarization direction is emitted from the PS composite element 104.
The light emitted from the PS composite element 104 passes through the condenser lens 105 and the field lens 106, and is radiated onto the liquid crystal panel 107. The divided light beams formed by the MLAs 102 and 103 are magnified at a magnification ratio determined on the basis of the focal length fc of the condenser lens 105 and the focal length fML2 of the microlenses 103M formed on the second MLA 103, and are radiated onto the entire incident surface of the liquid crystal panel 107. Accordingly, a plurality of magnified light beams overlap one another on the incident surface of the liquid crystal panel 107, thereby uniformly illuminating the incident surface of the liquid crystal panel 107. The liquid crystal panel 107 spatially modulates the light incident thereon in accordance with an image signal, and emits modulated light. The light emitted from the liquid crystal panel 107 is projected onto a screen (not shown) by the projection lens 108, so that an image is formed on the screen.
In liquid crystal panels, in order to form driving devices such as thin-film transistors (TFTs) on a substrate, a light-shielding area called a black-matrix is formed to separate adjacent pixels. Accordingly, aperture ratios of liquid crystal panels never reach 100%. Therefore, in liquid crystal panels of the known art, in order to increase the effective aperture ratio, one or more microlenses are arranged along an optical axis for each dot (a single pixel or a single sub-pixel), the microlenses being formed on an opposing substrate disposed at the light-incident side and serving as condenser lenses. The xe2x80x9ceffective aperture ratioxe2x80x9d is the ratio of light beams emitted from a liquid crystal panel to light beams incident on the liquid crystal panel. In projection liquid crystal display apparatuses, the effective aperture ratio is generally determined by taking into account not only the light loss caused in the liquid crystal panel but also the shading of light caused by the projection lens.
FIG. 13 is a diagram showing an example of the construction of the liquid crystal panel 107 in which microlenses are formed. In order to make the figure clear, the hatching is partly omitted. The liquid crystal panel 107 includes a pixel electrode substrate 140B and an opposing substrate 140A which is disposed at the light-incident side of the pixel electrode substrate 140B in such a manner that the opposing substrate 140A and the pixel electrode substrate 140B oppose each other with a liquid crystal layer 145 therebetween.
The pixel electrode substrate 140B includes a glass substrate 148, a plurality of pixel electrodes 146, and a plurality of black matrix elements 147. The pixel electrodes 146 and the black matrix elements 147 are arranged two-dimensionally on the glass substrate 148 at the light-incident side thereof. The pixel electrodes 146 are conductive, transparent members, and the black matrix elements 147 are formed between adjacent pixel electrodes 146. The black matrix elements 147 are shielded from light by, for example, a metal layer, and switching elements (not shown) used for selectively applying a voltage to the adjacent pixel electrodes 146 in accordance with an image signal are formed inside the black matrix elements 147. TFTs, for example, are used as the switching elements for applying a voltage to the pixel electrodes 146.
The opposing substrate 140A includes a glass substrate 141, a microlens array 142, and a cover glass 144 in that order from the light-incident side. A resin layer 143 is laminated between the glass substrate 141 and the microlens array 142. In addition, although not shown in the figure, opposing electrodes for generating a voltage between the pixel electrodes 146 and the opposing electrodes are arranged between the cover glass 144 and the liquid crystal layer 145. The resin layer 143 is formed of an optical plastic whose refractive index is n1.
The microlens array 142 is formed of an optical plastic whose refractive index is n2( greater than n1), and includes a plurality of microlenses 142M arranged two-dimensionally in correspondence with the pixel electrodes 146. The microlenses 142M are convex toward the light-incident side thereof and have positive refractive power. Each microlens 142M serves to condense light incident thereon through the glass substrate 141 and the resin layer 143 on the corresponding pixel electrode unit 146. When the projection lens 108 has a sufficient F-number, the light which is condensed by the microlenses 142M and passes though apertures 146A is utilized for displaying an image. When the microlenses 142M are provided, the amount of light that passes through the apertures 146A of the pixel electrodes 146 can be increased compared with a case in which the microlenses 142M are not provided. Accordingly, the effective aperture ratio can be increased and the light-utilizing efficiency can be improved.
In the liquid crystal panel 107 having the above-described construction, when a light component 211 whose divergence angle relative to an optical axis 200 is xcex2 is incident on one of the microlenses 142M, it is refracted by the power of the microlens 142M and is emitted in such a state that the divergence angle is increased compared with a case in which the microlenses 142M are not provided. The divergence angle of the emitted light (emission divergence angle), xcex8, is the sum of the angle a generated by the power of the microlens 142M and the initial angle xcex2. Accordingly, the following equation is given:
xcex8=xcex1+xcex2xe2x80x83xe2x80x83(1)
When fML is the focal length of the microlens 142M and is the external size (diameter) of the microlens 142M, the angle xcex1 generated by the power of the microlens 142M is defined as follows:
tan xcex1=a/fMLxe2x80x83xe2x80x83(2)
When fc and rc are the focal length and the radius, respectively, of the condenser lens 105 (see FIG. 12), the divergence angle of the illuminating light incident on the liquid crystal panel 107 (incident divergence angle), xcex2, is defined as follows:
xe2x80x83tan xcex2=rc/fcxe2x80x83xe2x80x83(3)
In addition, when the divergence angle of the light emitted from the liquid crystal panel 107 is xcex8, the projection lens 108 must have an F-number (Fno) defined as follows:
Fno=1/(2 sin xcex8)xe2x80x83xe2x80x83(4)
In the above-described liquid crystal panel 107, when light having a large divergence angle xcex2 is incident thereon, the microlenses 142M cannot sufficiently focus the light into the apertures 146A, so that the light is partly blocked by the black matrix elements 147. In addition, when the incident divergence angle xcex2 is large, the degree of divergence of the emitted light is increased by the power of the microlenses 142M compared with the case in which the microlenses 142M are not provided, and the emission divergence angle xcex8 is increased, as is clear from Equation (1). On the other hand, the projection lens 108 cannot receive light which is incident at an angle exceeding the angle determined by the F-number defined by Equation (4). Accordingly, shading occurs at the projection lens 108 when the emission divergence angle xcex8 is too large.
Accordingly, in order to improve the light-utilizing efficiency by using the microlenses 142M, the incident divergence angle xcex2 must be reduced. However, as is understood from Equation (3), in order to reduce the incident divergence angle xcex2, the focal length fc of the condenser lens 105 must be increased. In addition, the focal length of the microlenses 103M of the second MLA 103 must also be increased. Accordingly, when the incident divergence angle xcex2 is reduced, the optical path length from the light source 101 to the liquid crystal panel 107 is increased. When the optical path length is increased, the overall size of the apparatus is also increased and the light-utilizing efficiency in the overall illuminating optical system, that is, the system including the illuminating optical system positioned before the liquid crystal panel 107, is reduced. When a lens having an F-number corresponding to high brightness which is sufficient for the emission divergence angle xcex8 (for example, F-number=1.2 to 1.5) is used as the projection lens 108, shading at the projection lens 108 can be eliminated. However, there is a problem in that lenses having F-numbers corresponding to high brightness are difficult to design and thus high costs are incurred.
The problems of the above-described illuminating system and the microlenses 142M formed in the liquid crystal panel 107 can be summarized as follows:
(i) Light having a large incident divergence angle xcex2 causes shading at the black matrix elements in the liquid crystal panel or at the projection lens.
(ii) Although the effective aperture ratio of the liquid crystal panel can be increased by reducing the incident divergence angle xcex2, the light-utilizing efficiency of the overall illuminating system is reduced and the size of the apparatus is increased in such a case.
(iii) The divergence angle xcex8 of the light emitted from the liquid crystal panel is determined as the sum of the angle a generated by the power of the microlenses and the incident divergence angle xcex2, and is larger than that in the case in which the microlenses are not provided. Accordingly, a lens having an F-number corresponding to high brightness which is sufficient for the emission divergence angle xcex8 must be used as the projection lens. Such a projection lens is difficult to design and thus high costs are incurred.
The shading at the black matrix elements 147 described in (i) can be reduced by reducing the focal length of the microlenses 142M in the liquid crystal panel 107. However, in such a case, the angle a generated by the power of the microlenses 142M is increased, so that the emission divergence angle xcex8 is also increased. Accordingly, the problems described in (iii) occur. When the F-number of the projection lens 108 is reduced in order to increase the brightness, problems occur in that imaging performance is degraded and the size of the projection lens itself and the manufacturing costs are increased. In actual projection liquid crystal display apparatuses, the focal length fML of the microlenses 142M is increased and the distance between the pixel apertures and the microlenses is optimized in accordance with the F-number of the projection lens 108. Accordingly, the problems described in (i) and (ii) are not solved.
On the other hand, a liquid crystal panel shown in FIG. 14 has been suggested in which another microlens array 152 is disposed on the pixel electrode substrate 140B, and the angle a generated by the power of the microlenses 142M formed in the opposing substrate 140A is canceled when the light is emitted from the microlens array 152. In the example shown in FIG. 14, the microlens array 142 in the opposing substrate 140A is formed directly on the glass substrate 141 at the light-emission side thereof. In addition, another microlens array 152 formed of an optical resin is disposed on the pixel electrode substrate 140B at the light-emission side thereof. In addition, a glass substrate 151 is disposed on the microlens array 152 at the light-emission side thereof. The microlens array 152 includes a plurality of microlenses 152M which correspond to the microlenses 142M formed in the opposing substrate 140A. The microlenses 152M are convex at the light-emission side thereof and have positive refractive power. Each microlens 152M is constructed such that it serves as a collimator by being combined with the corresponding microlens 142M. When n1 and n2 are the refractive indexes of the glass substrate 141 and the microlenses 142M, respectively, and n3 and n4 are the refractive indexes of the microlenses 152M and the glass substrate 151, respectively, the liquid crystal panel is constructed such that n2 greater than n1 and n3 greater than n4 are satisfied.
When a light component 212, for example, is incident on the liquid crystal panel as shown in FIG. 14, it is refracted by an angle a by the power of the microlens 142M formed in the opposing substrate 140A. Then, the light component is refracted by the angle xe2x88x92xcex1 in the opposite direction by the corresponding microlens 152M formed on the pixel electrode substrate 140B due to the function thereof as a collimator. Accordingly, the angle a generated by the power of the microlens 142M formed in the opposing substrate 140A is canceled when it is emitted from the microlens 152M. Since the angle xcex1 is canceled, the emission divergence angle xcex8 is given by xcex8=xcex2 from Equation (1), and is reduced by the angle xcex1 compared with the example shown in FIG. 13. However, when the microlenses are arranged as described above, if, for example, a light component 213, whose incident divergence angle is xcex2 and which must be incident on a microlens 152M-1, is incident on the adjacent microlens 152M-2, the microlens 152M-2 cannot serve as a collimator for this incident light component. In such a case, the above-described relationship (xcex8=xcex2) cannot be obtained and the emission divergence angle xcex8 becomes larger than the incident divergence angle xcex2, so that the effective aperture ratio cannot be increased.
In addition, Japanese Unexamined Patent Application Publication No. 5-341283 discloses a liquid crystal panel in which the incident divergence angle xcex2 is canceled. The liquid crystal panel disclosed in this publication includes a pair of glass substrates and a liquid crystal layer disposed between the glass substrates, and microlenses are arranged on both sides of at least one of the glass substrates in correspondence with pixel apertures. In this liquid crystal panel, the focal length of the microlenses formed at one side of the glass substrate is made the same as the focal length of the microlenses formed at the other side of the glass substrate. In addition, the distance between the microlenses formed at one side of the glass substrate and the microlenses formed at the other side of the glass substrate is made the same as the focal length. When collimated light is incident, the microlenses at either side of the glass substrate serve to converge the light on the surface at the other side. Thus, the incident divergence angle xcex2 is canceled before the light is emitted. According to this publication, the microlenses are formed by the ion-exchange method.
In the above-described publication, the microlenses at either side of the substrate are convex toward the inside and the surfaces facing outward (surfaces at both sides of the substrate) are flat. In addition, the distance between the microlenses formed at the side closer to the pixel apertures and the pixel apertures is approximately 0. In this case, the thickness of the substrate including the microlenses is about several tens of micrometers. However, in the above-described construction, there is a problem in that the substrate including the microlenses is extremely difficult to manufacture. Especially when the ion-exchange method is applied, it is difficult to control the thickness, and it is also difficult to process a thin substrate whose thickness is several tens of micrometers at a high precision so as to obtain the desired optical characteristics. For example, although the lens surfaces of the microlenses formed at both sides of the substrate must be polished in order to obtain the desired optical characteristics, it is extremely difficult to polish a thin substrate whose thickness is several tens of micrometers. In recent years, high-precision liquid crystal panels with small pixel pitches have been required, so that high processing precision is necessary. Accordingly, the liquid crystal panel according to the above-described publication has a disadvantage in this point.
In consideration of the above-described problems, an object of the present invention is to provide a liquid crystal display device and a projection liquid crystal display apparatus in which the effective aperture ratio can be increased and the light-utilizing efficiency can be improved without increasing the size or complicating the manufacturing process. In addition, another object of the present invention is to provide a liquid crystal display device and a projection liquid crystal display apparatus in which the light-collection efficiency is optimized by adjusting the positional relationship between the microlenses and the pixel apertures along the optical axis.
In order to attain the above-described objects of the present invention, a liquid-crystal display according to the present invention includes a liquid crystal layer; a plurality of pixel electrodes, having pixel apertures which allow light to pass therethrough; and at least one microlens array disposed at at least one of a light-incident side and a light-emission side of the liquid crystal layer, the microlens array having a plurality of microlenses arranged two-dimensionally in correspondence with the pixel apertures. Each microlens includes a condenser lens and a field lens, the condenser lens having at least one lens surface along an optical axis and condensing light incident thereon toward the corresponding pixel aperture, and the field lens having at least one lens surface along the optical axis and being constructed such that the focal point of the field lens is at approximately the same position as the principal point of the condenser lens. In addition, the overall focal point of the condenser lens and the field lens is shifted from the corresponding pixel aperture and the amount of shift is set such that an effective aperture ratio is increased compared with the case in which the overall focal point is at the same position as the corresponding pixel aperture. Preferably, the amount of shift exceeds xc2x110% of the overall focal length. In addition, preferably, when light having a divergence angle component is incident, the divergence angle component is eliminated by the optical behavior of the field lens when the light is emitted from the microlens array, so that the emission angle of the incident light is the same as an emission angle of a principal ray which is incident parallel to the optical axis. The liquid crystal display device may be used in a projection liquid crystal display apparatus in which light that has passed through the liquid crystal display device is projected by a projection lens. In such a case, a numerical aperture of each microlens is set such that the numerical aperture approximately corresponds to an F-number of the projection lens.
In addition, according to the present invention, a projection liquid crystal display apparatus includes a light source which emits light; a liquid crystal display device which serves to modulate incident light; and a projection lens which projects light modulated by the liquid crystal display device. The liquid crystal display device includes a liquid crystal layer, a plurality of pixel electrodes, having pixel apertures which allow light to pass therethrough, and at least one microlens array disposed at at least one of a light-incident side and a light-emission side of the liquid crystal layer, the microlens array having a plurality of microlenses arranged two-dimensionally in correspondence with the pixel apertures. Each microlens includes a condenser lens and a field lens, the condenser lens having at least one lens surface along an optical axis and condensing light incident thereon toward the corresponding pixel aperture, and the field lens having at least one lens surface along the optical axis and being constructed such that the focal point of the field lens is at approximately the same position as the principal point of the condenser lens. In addition, the overall focal point of the condenser lens and the field lens is shifted from the corresponding pixel aperture and the amount of shift is set such that an effective aperture ratio is increased compared with the case in which the overall focal point is at the same position as the corresponding pixel aperture.
In the liquid crystal display device and the projection liquid crystal display apparatus according to the present invention, each microlens is constructed of a condenser lens and a field lens. The condenser lens serves to condense light emitted from the light source and incident on the condenser lens toward the corresponding pixel aperture, and the field lens is constructed such that the focal point thereof is at approximately the same position as the principal point of the condenser lens. In such a construction, when light having a divergence angle component relative to an optical axis is incident on the microlens, the divergence angle component is eliminated when the light is emitted therefrom. Accordingly, even when the focal length of the microlens is reduced, the divergence angle of the emitted light can be prevented from being increased. When the liquid crystal display device is used in a projection liquid crystal display, shading of light due to projection lens can be reduced. In addition, according to the present invention, the pixel aperture is shifted from the overall focal point of the condenser lens and the field lens, and the amount of shift is set such that the effective aperture ratio is increased compared with the case in which the overall focal point is at the same position as the corresponding pixel aperture. When all of the angular components of the incident light are analyzed, the effective aperture ratio is not always optimum when the overall focal point is at exactly the same position as the pixel aperture. When all of the angular components are taken into account, the effective aperture ratio is increased when the overall focal point of the microlens is shifted away from the pixel aperture. Accordingly, the positional relationship between the overall focal point and the pixel aperture is preferably optimized so that the effective aperture ratio can be increased. The effective aperture ratio shows the ratio of the light beams passing through the microlens, the pixel aperture, and the projection lens to the light beams emitted from the light source and incident on the pixel.
Thus, according to the liquid crystal display device and the projection liquid crystal display apparatus of the present invention, the effective aperture ratio can be increased and the light-utilizing efficiency can be improved without increasing the size or complicating the manufacturing process. Thus, the light-utilizing efficiency can be improved and the optical output can be increased, and the size of the projection liquid crystal display apparatus and the cost of the projection lens can be reduced. Furthermore, an allowable displacement between the substrate in which the pixel apertures are formed and the substrate in which the microlenses are formed can be increased.